The major role of vascular networks in the circulatory system is to transport blood, oxygen, nutrients, hormones, and cellular waste to and from various organs to maintain biological homeostasis. Accordingly, data points related to the circulation of blood flow through the vasculature are important physiological parameters that dictate biological transport phenomena and often have critical implications for vascular disease and medical diagnosis.
A study of the circulation requires an understanding of not only hemodynamics (blood flow), but also the vasculature's morphological (e.g., diameter, length, volume, etc.) and topological (e.g., connectivity patterns) information, and any potential structure-function relations thereof. Functionally, the vasculature structure serves metabolism where there is an intimate structure-function relation. Indeed, vascular patterns have been used as a basis to elucidate the origin of allometric scaling laws (e.g., the scaling law of metabolism, which can be used to predict structural and functional properties of vertebrate cardiovascular and respiratory systems based on principles of maximizing metabolic capacity and preserving energy dissipation) and various intraspecific scaling laws (e.g., volume-diameter, flow-length, and length-volume relationships and the scaling law of flow resistance).
The mean transit time (MTT), which is the time required to transport blood within a vascular network, plays a vital role in the physiological function of a circulatory system. The vascular network has structure heterogeneity and complexity with respect to the spatial arrangement of vessels, as well as the ability to adapt its anatomy in response to hemodynamic and metabolic stimuli. Accordingly, development of structure-function relationships that relate MTT to vascular morphology are fundamental to understanding the interplay between vascular form and function and, thus, provide a better rationale for clinical diagnostics and therapies. Because this has not yet been achieved, conventional measurement of MTT relies on quantification of blood volume and flow rate, both of which are challenging to accurately determine—particularly in small vessels. Especially considering that MTT is such a seminal physiological parameter with respect to biological transport, it would be beneficial to provide a framework for an accurate and non-invasive way to determine MTT in various species and organs throughout the vasculature.